What causes pond algae?

Pond AlgaeAlgae: It's green, gross and one of your pond's most unwelcome guests.

But what is it exactly, and how did it get there?

“Algae” describes a diverse group of organisms that thrive in warm, nutrient-rich environments like the shallow water of a pond. A well-built pond will prevent algae from going crazy by creating an environment where beneficial bacteria and plants can thrive and consume all of the nutrients algae would need to grow.

Every pond should have at least a little bit of the green stuff in it. Short of chlorinating your pond and killing all your fish and plants, there's no way to keep it out entirely - and if you want to go to that extreme, you might as well just have a swimming pool. Luckily, a moderate amount of algae won't harm your fish, so getting rid of it has more to do with aesthetic preference than anything else. Many pond owners outside of the U.S. even embrace the look of green water.

If green isn't your color, you have plenty of options for keeping the gunk at bay.  Let's start by talking a little more about how it ends up in your pond in the first place.


At a Glance: Causes of Pond Algae

  • Too little beneficial bacteria
  • Too few plants
  • Excess nutrients (i.e. fertilizer, decaying fish food)
  • Warm, shallow water
  • Lack of an established ecosystem in new ponds
  • Seasonal changes

Green Water or Stringy Goop?

 
Pond owners typically worry about two broad categories of algae: single-cell and string. Single-cell algae is, as the name implies, very tiny and turns your normally crystal-clear water murky, pea-soup green. String algae, on the other hand, is green, stringy and you can pick it up with your hands. Both types of algae can prove a burden for clear-water-loving pond owners but differ in their causes and treatments.

Single-cell Algae

Green Pond WaterSingle-cell algae is also known as phytoplanktonic or free-floating algae. These organisms - which are so tiny that you can't see the individual alga - feed on nitrites, a byproduct of beneficial bacteria breaking down harmful ammonia. Green water means an outbreak of single-cell algae, which means you have too many nitrites in your pond.

So how does that happen? Single-cell algae tends to plague pond owners the most in the spring, when algae thrives in warming water and beneficial bacteria colonies are working to re-establish themselves after their winter slumber. Without the bacteria, algae don't have to compete for nitrites - which means they can reproduce in extreme numbers and turn water green. The lack of an established ecosystem also creates green water problems for owners of newly built or renovated ponds, as well as people who get overzealous when cleaning their water features.

String Algae

How to get rid of string algae

String algae has lots of names and variations: benthic algae, attached algae, horsehair algae, water net, blanket weed. When most people talk about string algae, though, they're referring broadly to any type of green, stringy stuff that you can pick up with your hands. Like green-water algae, it won't directly hurt your pond fish, but it can start to cause problems if too much of it decays and creates excessive ammonia in your pond.

The main cause of string algae is having too few plants in a pond. Unlike single-cell algae, string algae feeds on nitrates, not nitrites. These nitrates fill up your pond as a byproduct of bacteria consuming nitrites. Your pond plants also enjoy consuming nitrates, so adding more will help take away the string algae's food source and starve it out of existence. 

Algae Treatment and Prevention

The pond supply market offers all kinds of treatments for algae: algaecides, barley straw and UV clarifiers, just to name a few. We usually recommend taking a natural approach.

 

The best way to prevent string algae is to plant more and a wider variety of plants in your pond because different types of plants will eat different kinds of nitrates. An iris will suck up one kind, and a lily will consume another. A blue flower will chow down on one, and a pink flower yet another. A plant that blooms in May will eat a different kind than one that blooms in September. If you find yourself with a bunch of string algae clinging to your rocks or floating in your pond, remove as much as you can with your hands or a net, then stock up on plants. 

Find pond plant ideas here

 

The best way to get rid of single-cell algae, on the other hand, is to introduce more beneficial bacteria, which will eat the nitrites before the algae has a chance to grow. Beneficial bacteria is relatively inexpensive - you can get enough to treat 10,000 gallons of water for under $20 - and will not harm your fish, plants, pets or wildlife. Most formulas are more or less the same, but you can find varieties specifically marketed for cold-water conditions, new ponds or warm water. Other kinds combine bacteria with other water treatments. Using beneficial bacteria is as easy adding a few pumps to your pond each week, or buying an autodoser to do it for you.

Adding shade to your pond can also help prevent algae. You can do this by planting trees nearby, adding shade-producing pond plants like waterlilies or even buying special pond water tints that prevent sunlight from reaching algae.

Reducing the amount of algae-feeding nutrients to your pond will help too. Take care not to let fertilizer run into your pond from the surrounding yard, and don't overfeed your fish to the point where excess food is sinking to the bottom of your pond.

Algae Treatments You Probably Don't Need

If you find yourself battling algae, we will usually recommend looking for a natural solution first: adding bacteria to starve single-cell algae, or adding plants to starve string algae. Algaecides, UV clarifiers and other products might also play a role in your fight, but dealing with the root of the problem can prevent other issues from arising in the future.

What's wrong with algaecides? Nothing, necessarily. We sell several varieties in our store, and the people who choose to use them are generally pleased with the fast results. Algaecide can, however, kill fish or pond plants if used in excess. That means pond owners have to carefully calculate the number of gallons of water in their pond and meticulously follow the dosing instructions on the bottle. An accidental spill or miscalculation can lead to a pond full of dead fish. Algaecides also don't treat the underlying cause of your green water or string algae, so you could find yourself back at square one not long after adding treatments.

We don't recommend UV clarifiers or sterilizers for treating algae either.  Like algaecides, there's nothing inherently wrong with these products, and some people swear by them. And unlike algaecides, they won't even endanger your pond life.

Clarifiers and sterilizers work by circulating your water past a UV light. The energy from the light penetrates the cell wall of single-cell algae, muddling its DNA and making it unable to reproduce. Clarifiers move the water through their pipes at a fairly quick pace, making them more efficient for treating large bodies of water, whereas sterilizers move more slowly in order to kill harmful kinds of bacteria and viruses.

So why don't we recommend UV clarifiers and sterilizers? We simply believe you don't need them. We have never, in our 30 years of pond-building experience, installed one in a pond and still have no trouble keeping algae at bay using beneficial bacteria and plants. These treatments also don't do anything to treat string algae, which stubbornly clings to your rocks instead of circulating past the UV light.

The Bottom Line

If you have string algae, add more plants to your pond. If you have green water, add beneficial bacteria. Be careful using any other products marketed for killing algae.

Learn More about Pond Ecosystems